//SamXIAO
#include <bits/stdc++.h>
using namespace std;
#define PI 3.1415926
#define   SQUARE(x) ((x)*(x))
//double SQUARE(double x) { return x*x;}
#define N_MAX 987654321
#define N_MIN -N_MAX
int f[286];
char ss[1086];
int pos[2020];
void w1()
{
	int cnt = 0, ans = 0;
	char ch = getchar();
	while('a' <= ch && ch <= 'z'){
		f[ch]++;
		cnt++;
		if(!pos[ch]) pos[ch] = cnt;
		ch = getchar();
	}
	for(int i = 'a'; i <= 'z'; i++)
		if(1 != f[i]) pos[i] = 0;

	int mi = 9999 ;
	for(int i = 'a'; i <= 'z'; i++)
		if(pos[i] && pos[i] < mi) ans = i, mi = pos[i];
	
	if(ans) printf("%c", ans);
	else printf("no");
}

int main()
{
	w1();
  return 0;
}
